In the
reactor plant, the principle source of radiation comes from the reactor core. Attenuationof
this radiation is performed by shielding
materials located around the core. This chapter discusses the various materials
used in a reactor plant for shielding.

EO 1.11 DESCRIBE
the requirements of a material used to shield against the following types of
radiation:

a. Beta c. High energy neutrons

b. Gamma d. Low energy neutrons

Overview

Shielding design is relatively straightforward depending
upon the type of radiation (gamma, neutron, alpha, beta). For example, when
considering the reactor core, it is first necessary to slow down the fast
neutrons (those not directly absorbed) coming from the core to thermal energy
by utilizing appropriate neutron attenuating shielding materials that are
properly arranged. This slowing down process is mostly caused by collisions
that slow the neutrons to thermal energy. The thermal neutrons are then
absorbed by the shielding material. All of the gamma rays in the system, both
the gamma rays leaving the core and the gamma rays produced by neutron
interactions within the shielding material have to be attenuated to appropriate
levels by utilizing gamma ray shielding materials that are also properly
arranged. The design of these radiation shields and those used to attenuate
radiation from any radioactive source depend upon the location, the intensity,
and the energy distribution of the radiation sources, and the permissible
radiation levels at positions away from these sources. In this chapter, we will
discuss the materials used to attenuate neutron, gamma, beta, and alpha
radiation.

The shielding of neutrons introduces many complications
because of the wide range of energy that must be considered. At low energies (less
than 0.1 MeV), low mass number materials, such as hydrogen in H2O, are
best for slowing down neutrons. At these energies, the cross section for
interaction with hydrogen is high (approximately 20 barns), and the energy loss
in a collision is high. Materials containing hydrogen are known as hydrogenous
material, and their value as a neutron shield is determined by their hydrogen
content. Water ranks high and is probably the best neutron shield material with
the advantage of low cost, although it is a poor absorber of gamma radiation.

Water also provides a ready means for removing the heat
generated by radiation absorption. At higher energies (10 MeV), the cross
section for interaction with hydrogen (1 barn) is not as effective in slowing
down neutrons. To offset this decrease in cross section with increased neutron
energy, materials with good inelastic scattering properties, such as iron, are
used. These materials cause a large change in neutron energy after collision
for high energy neutrons but have little effect on neutrons at lower energy,
below 0.1 MeV.

Iron, as carbon steel or stainless steel, has been
commonly used as the material for thermal shields. Such shields can absorb a
considerable proportion of the energy of fast neutrons and gamma rays escaping
from the reactor core. By making shields composed of iron and water, it is
possible to utilize the properties of both of these materials. PWRs utilize two
or three layers of steel with water between them as a very effective shield for
both neutrons and gamma rays. The interaction (inelastic scattering) of high
energy neutrons occurs mostly with iron, which degrades the neutron to a much
lower energy, where the water is more effective for slowing down (elastic
scattering) neutrons. Once the neutron is slowed down to thermal energy, it
diffuses through the shield medium for a small distance and is captured by the
shielding material, resulting in a neutron-gamma (n)
reaction. These gamma rays represent a secondary source of radiation.

Iron turnings or punchings and iron oxide have been
incorporated into heavy concrete for shielding purposes also. Concrete with
seven weight percent or greater of water appears to be adequate for neutron
attenuation. However, an increase in the water content has the disadvantage of
decreasing both the density and structural strength of ordinary concrete. With
heavy concretes, a given amount of attenuation of both neutrons and gamma rays
can be achieved by means of a thinner shield than is possible with ordinary
concrete. Various kinds of heavy concretes used for shielding include barytes
concrete, iron concrete, and ferrophosphorus concrete with various modified
concretes and related mixtures. Boron compounds (for example, the mineral
colemanite) have also been added to concretes to increase the probability of
neutron capture without high-energy gamma-ray production.

Boron has been included as a neutron absorber in various
materials in addition to concrete. For example, borated graphite, a mixture of
elemental boron and graphite, has been used in fast-reactor shields. Boral,
consisting of boron carbide (B4C) and
aluminum, and epoxy resins and resin-impregnated wood laminates incorporating
boron have been used for local shielding purposes. Boron has also been added to
steel for shield structures to reduce secondary gamma­ray production. In
special situations, where a shield has consisted of a heavy metal and water, it
has been beneficial to add a soluble boron compound to the water.